The Boomerang Nebula is a cloud of gas and dust blown off by a dying star, about 5,000 light-years from Earth in the constellation Centaurus. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. The nebula's two lobes stretch about 2 light-years, or 19 trillion kilometers, from end to end. As of 2026 it is the coldest known natural object in the universe.

Size

Seen through the Hubble Space Telescope, the nebula has two nearly matching lobes on opposite sides of its central star, giving it the shape of an hourglass. NASA estimates that each lobe is nearly 1 light-year long, which makes the whole nebula about 2 light-years long.

That figure depends on the nebula's distance, which is uncertain. NASA gives about 5,000 light-years, and the team that studied it with the ALMA radio observatory uses a similar figure, about 4,900. The team also calculated that it can be no more than about 13,000 light-years away. If it is farther than 5,000 light-years, it is also larger.

The visible lobes are only part of the nebula. ALMA's observations traced very cold gas flowing out from the star to at least 120,000 astronomical units, where 1 astronomical unit (au) is the average distance from Earth to the Sun. That makes this cold outflow at least 3.8 light-years across, nearly twice the length of the visible nebula. Astronomers think the lobes are the parts of it lit by starlight escaping from the center.

Size comparisons

• The nebula is about half as long as the distance from the Sun to Proxima Centauri, 4.2 light-years.
• A parsec, a unit of distance equal to about 3.26 light-years, is about 1.6 times the nebula's length.
• A dense band of dust and gas circles the star's middle. It is about 1,740 au across, about 29 times the width of the orbit of Neptune.
• If Neptune's whole orbit were shrunk to the size of a basketball, that dusty band would be about 7 meters (23 feet) across, and the visible nebula about 500 meters (1,600 feet) long.

The coldest known place

Radio measurements made in 1995 showed that gas in the nebula is only about 1 kelvin, one degree above absolute zero. That is about −272 degrees Celsius (−458 degrees Fahrenheit). It is even colder than the cosmic microwave background, the oldest light in the universe, which reaches Earth from the whole sky at about 2.7 kelvins.

The gas has cooled by expanding very fast. It streams away from the star at about 164 kilometers per second (590,000 kilometers per hour). ALMA observations show that the cold outflow holds at least 3.3 times the mass of the Sun.

A dying star

The Boomerang is a pre-planetary nebula. That is a short stage, lasting about 1,000 years, between an old, swollen red giant star and a planetary nebula, the glowing shell of gas a dying star leaves behind. The gas is the red giant's outer layers.

The star has lost mass unusually fast. Astronomers studying it with ALMA argue that a companion star spiraled into the red giant and merged with its core, flinging off the giant's outer layers. Keith Taylor and Mike Scarrott named the nebula in 1980, after its shape as seen through a large telescope in Australia.

Sources

• Hubble Catches Scattered Light from the Boomerang Nebula. NASA, 2005.
• ALMA Observations of the Coldest Place in the Universe: The Boomerang Nebula. Sahai et al., The Astrophysical Journal, 2013.
• The Coldest Place in the Universe: Probing the Ultra-Cold Outflow and Dusty Disk in the Boomerang Nebula. Sahai et al., The Astrophysical Journal, 2017.
• WMAP Overview. NASA Science.
• The International System of Units (SI). In SI Brochure, 9th edition. Bureau International des Poids et Mesures, 2019.
• Webb captures detailed beauty of Ring Nebula in high definition. ESA/Webb, 2023.
• Measuring the Universe. International Astronomical Union.
• Proxima's orbit around Alpha Centauri. Kervella et al., Astronomy & Astrophysics, 2017.
• Neptune Fact Sheet. NASA, 2024.
• Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.